1. Field of the Invention
The present invention relates to a 3D image display apparatus for viewing images through shutter glasses and a control method for such an apparatus.
2. Description of the Related Art
A 3D image display apparatus based on a frame-sequential method is known, in which a three-dimensional image (a 3D image) is composed by alternately displaying a right eye image and a left eye image having parallax, and causing different images to be viewed by the right eye and the left eye through shutter glasses (see Japanese Patent Application Publication No. 2000-275575). Shutter glasses mainly use a liquid crystal type of shutter, in which liquid crystal shutters each comprising a mutually superimposed liquid crystal element plate and polarizing plate that perform a shutter opening and closing operation, are provided for the left eye and the right eye.
An ideal relationship between the transmission response characteristics of the shutter glasses and the display timing of an image in a 3D image display apparatus based on a frame-sequential method is described here with reference to FIG. 9A, FIG. 9B and FIG. 9C. FIG. 9A shows the transmission response characteristics (the temporal change in transmittance (transmissivity)) of ideal shutter glasses; the horizontal axis represents time and the vertical axis represents the transmittance of the shutter glasses. The solid line 12 indicates the transmission response characteristics of the left-eye shutter and the dotted line 13 indicates the transmission response characteristics of the right-eye shutter. It is known that there exists a delay caused by the response speed of the liquid crystals, in the opening and closing of the shutter. The diagonal portions from “open” to “close” and “close” to “open” in the solid line 12 and the dotted line 13 indicate this delay. FIG. 9B shows a schematic view of a left eye image and a right eye image, in which the horizontal axis represents time and the vertical axis represents the display brightness. The solid line 14 shows the light emission characteristics for the left eye, and the diagonally hatched portion is a schematic view of the brightness of the left eye image. The dotted line 15 shows the light emission characteristics for the right eye, and the diagonally hatched portion is a schematic view of the brightness of the right eye image. In order to simplify this description, FIG. 9B shows an example in which an image of uniform brightness is displayed.
FIG. 9C shows the brightness in a case where an observer observes an image having the characteristics shown in FIG. 9B, through shutter glasses having the characteristics in FIG. 9A. The vertical axis in FIG. 9C shows the observed brightness. In the light emission time (t2 to t3) of the left eye image 14, the left eye shutter assumes an open state, the right eye shutter assumes a closed state, and therefore the light 16 of the left eye image is transmitted by the left eye shutter only. Conversely, in the light emission period of the right eye image, the light 17 of the right eye image is transmitted only by the right eye shutter. Therefore, the observer observes only the left eye image with the left eye, and observes only the right eye image with the right eye.
If the shutter glasses have the ideal transmission response characteristics shown in FIG. 9A, then as shown in FIG. 9C, the display image is observed correctly by the observer. However, the actual transmission response characteristics of a liquid crystal shutter based on a TN method or an STM method which is generally used in shutter glasses do not conform to the ideal characteristics shown in FIG. 9A. Moreover, based on experimentation by the present inventors, it was found that the transmission response characteristics of a liquid crystal shutter vary with the wavelength (color) of the transmitted light, as described below.
FIG. 10A shows the results obtained when respective monochromatic images of R, G and B are displayed using an image display apparatus based on three primary light emission colors of R (red), G (green) and B (blue), and measuring change in the light transmittance of the liquid crystal shutter in respect of each color. FIG. 10A shows the transmission response characteristics in relation to R light, G light and B light, sequentially from the top part of the diagram, in which the horizontal axis represents time and the vertical axis represents the transmittance. The time t1 indicates the timing at which the shutter opens, time t3 indicates the timing at which the shutter closes, and t2 to t3 indicates one field period.
It is known in relation to the transmission response characteristics 20 with respect to R light that the transmittance in the first half is low, and that transmittance of only approximately 50% of the maximum transmittance is obtained at the start time t2 of one field period. In the transmission response characteristics 21 with respect to G light, the transmittance in the first half is low, and although the transmittance is slightly higher than the transmittance of R light, transmittance of only approximately 60% of the maximum transmittance is obtained at the time t2. Furthermore, the transmission response characteristics 22 in relation to B light are the reverse of R light and G light, in that the transmittance rises quickly, but then falls in the second half and at time t3, becomes approximately 70% of the maximum transmittance. These differences in transmittance depending on the color are thought to be caused by wavelength dispersion.
FIG. 10B shows the brightness in a case where an observer observes an image 14 having the characteristics shown in FIG. 9B, through shutter glasses having the characteristics in FIG. 10A. Even if image display is carried out at a uniform brightness throughout one field period, the R image and the G image appear dark in the first half of the period and the B image appears dark in the second half of the period. Therefore, if a white image (an image in which the RGB brightness ratios are the same) is displayed on a hold-driven liquid display monitor, for example, a phenomenon occurs in which the RGB brightness ratio changes over time within one field period. More specifically, at the start of the one field period, a slightly cyan bluish white image is observed, the image gradually becomes pure white, and finally, changes to a slightly yellowish white. However, since temporal change in color within an extremely short period of time such as one field period is practically impossible to observe with the naked eye, this phenomenon has not presented a problem in conventional 3D display apparatuses using a hold-driven liquid crystal display.
However, in the case of a 3D image display apparatus using a display monitor based on a drive method which emits light in a line-sequential or block-sequential manner, variations in the transmission response characteristics of the liquid crystal shutter glasses depending on the wavelength can have a great effect on the quality of the displayed image. The problems which occur in a display using line-sequential light emission are now described with reference to FIG. 9D. In the case of 1080p HDTV, 1080 scanning lines emit light sequentially from the top to the bottom of the screen in one field period. Consequently, when the display image in FIG. 9D is observed through shutter glasses having the characteristics in FIG. 10A, the line 18 at the upper end of the screen which emits light first appears to be slightly cyan compared with the central portion of the screen, and the line 19 at the lower end of the screen which emits light last appears to be slightly yellow compared with the central portion of the screen. In this way, in the case of a display based on line-sequential light emission, variations in the transmission response characteristics depending on the wavelength are observed as spatial fluctuations in color and brightness, and therefore are liable to be recognized as decline in the display quality. A similar problem occurs in the case of a display based on a method which divides a screen into a plurality of blocks and causes the blocks to emit light sequentially (block-sequential light emission).
One example of a display using line-sequential emission is a field emission display (FED), or the like. Furthermore, examples of a display using block-sequential emission are a field emission display, a backlight scanning liquid crystal display, an organic electroluminescence display, and the like.